The present invention relates generally to decoding of received signals and, more particularly, to a method for compensating soft values that are input to a decoder to account for bias attributable to bit width limitations.
The ultimate purpose of a communication system is to transmit information from an information source to a destination over a communication channel. In wireless communication systems, noise and multipath fading cause bit errors to occur during transmission. Forward error correction (FEC) may be employed to significantly reduce the number of bit errors. An encoder at the transmitter adds redundancy to information prior to its transmission. At the receiver, a decoder makes use of the redundant information and a priori knowledge of the coding scheme to detect and correct errors that may have occurred during transmission of the information.
In some communication systems, the received signal is demodulated by the receiver to generate soft values for input to the decoder rather than hard bit decisions. The soft values contain reliability information that can be exploited by the decoder to improve decoding. The number of bits used to represent the soft values, hereinafter referred to as the bit width, is one consideration in the design of a soft value decoder. It is usually desirable to limit the bit width of the soft values to reduce the computational complexity of the decoder. However, limiting the bit width of the soft values may degrade the performance of the decoder.
The channel decoder is typically designed based on the assumption that the noise will have a zero mean. When the bit width is limited, many of the soft values may become saturated. If the number of saturated values is unevenly matched between the positive and negative values, the noise introduced by the communication channel cannot be assumed to have zero mean. When the positive saturated values exceed the negative saturated values, the mean of the noise will be low. Conversely, when the negative saturated values exceed the positive saturated values, the mean of the noise will be high. If the bias introduced by limiting the bit width of the soft values is not compensated for, the performance of the decoder may be degraded.